The family of missing Boronia schoolgirl "Bung" Siriboon are clinging to hope she's alive and being held against her will, more than two years after the 13-year-old vanished while walking to school.

Bung's stepfather Fred Pattison stood alongside homicide squad detectives to announce a $1 million reward and the chance of immunity from prosecution for information about her disappearance on Tuesday.

"She's coming home, there's someone out there who won't' let her go, or for some reason she can't get home," Mr Pattison said. "You can't believe anything else.''

Siriyakorn "Bung" Siriboon has been missing since 2011. Photo: Supplied

He said Bung's mother Vanidda has received ongoing advice from Buddhist monks and mediums that their daughter was still alive.

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"Being a Buddhist my wife goes to the temple and talks to monks and things like that and they still, every medium or card reader, or person that we believe in and trust have all said the same thing that she's alive somewhere," Mr Pattison said.

Detectives continue to investigate two men of interest, but believe a member of the public may hold a vital piece of information that could close the 2½-year case.

Dressed in her blue and white school uniform Bung left her home on Elsie Street, Boronia, about 8.30am on June 2, 2011.

The last confirmed sighting of her was moments later when a neighbour saw her walking towards Albert Avenue.Bung, a slim girl of Thai origin, had only been in Australia four years before she vanished.

Police have investigated more than 250 registered sex offenders, searched parklands and followed up on more than 1200 pieces of information.

One man who has not been ruled out as a suspect is a 24-year-old who last year claimed he accidentally killed Bung with his car, then panicked and disposed of her body in a nearby park.

He was interviewed by police in August and October, but has never been charged.

"Aspects of what he's told us have been closely examined," Homicide Squad Detective Inspector John Potter said.

"At this stage we're not in a position to lay any charges in regards to what he's told us, but he does remain a person of interest in this case."

In August, police conducted an extensive search of Old Joe's Creek Reserve, but no trace of the missing girl was found.

Other suspects have been brought to the attention of police by callers to Crime Stoppers or through analysis of the sex offenders' register.

"There have been a number of these people and one other remains a person of interest to us, we're not able to rule him out either," Detective Inspector Potter said.

He said it was possible vital information had been missed in the early days of the investigation. The homicide squad took over the week after Bung was reported missing.

"We think somebody out there has the piece of information that's crucial to solving this case," he said.

"One million dollars is a life-changing amount of money," he said.On Tuesday Bung's mother Vanidda stood before the cameras and clung to her husband's hand but overcome by emotion she was unable to speak.

"We've tried to keep her away from thinking about this everyday," Mr Pattison said.

"It's a very big deal having to try and deal with this."

Mr Pattison said the family have had their lives on hold since Bung's disappearance and would continue to wait for her.

"It's like living in limbo," he said. "Plans get put on hold, some days are more difficult than others ... it's time for somebody to come out and say something.

"It's hard to believe that someone can just vanish off the face of the earth at half past eight in the morning on the way to school."

A reward of up to $1 million will be paid at the discretion of the Chief Commissioner for information that leads to the arrest and conviction of the person or persons responsible.

The Department of Public Prosecutions will also consider granting indemnification from prosecution.